This invention is concerned with novel heat-sealable polyurethane compositions and with the products prepared from them. More particularly, it is concerned with, preferably flexible, polyurethane cellular products which are heat-sealable to other materials to form novel and useful laminates and the laminates so produced.
Polyurethane foams are widely used. They are generally prepared by the reaction of at least one active hydrogen-containing compound (called a polyol) and at least one polyisocyanate, in the presence of at least one blowing agent such as water, and often at least one reaction catalyst and, optionally, at least one foam stabilizer other additive or combination thereof. The cellular polymer structure of a polyurethane foam has a skeletal framework of relatively heavy strands forming an outline for the cell structure. The skeletal framework strands are connected by very thin membranes, often called windows, which form the cell walls. In open-celled foams, some of the windows are open or torn in each cell, thus forming an interconnecting network open to fluid flow (liquid or gas).
In the preparation of many articles which employ foams, it is necessary to form a bond or adhesion between the foam and a substrate such as wood, textile, metal, foil, another polymer (for instance in the form of a film or another foam) and the like. It is preferred that adhesives not be used between layers because adhesives generate VOC (Volatile Organic Compound), may stiffen over time, affect appearance or function, or add expense. One foam joining method without glue or adhesive is heat bonding, also called heat laminating or heat sealing. A surface of a foam layer is heated to a temperature sufficient to locally melt the foam. The foam layer is then joined to a substrate while the surface is melted. Upon cooling, the melted surface solidifies to form a bond or adhesion between the foam and the substrate. While resolidification may be referred to as “hardening” it is usually preferred that a layer hard to the touch is avoided. The resulting bond between the foam and substrate is frequently as strong or stronger than the strength of the original foam. Heat bonding includes such processes as flame bonding or lamination and ultrasound or other high frequency bonding, lamination or welding. In case of high frequency welding it is required that the foam exhibits a dielectric loss factor of at least 0.01 at 1 MHZ (Megahertz) in order to warm upon exposure to high frequency alternative fields, hence additives containing polar groups are usually required to maximize these foam characteristics (see Polyurethane Handbook, by G. Oertel, Hanser publisher, section 3.4.11).
Use of many polyurethane foams are severely restricted in certain applications since they are unsuitable for heat bonding, especially flame bonding or high frequency (HF) welding. Such applications include manufacture of composite systems having foams with surface layers of another material such as a foil or fabric. Such composite systems are, in many cases, contoured and/or shaped by high frequency welding. In general, polyurethanes wherein the polyol component is primarily a conventional polyether polyol, made by alkoxylation of a starter containing a plurality of reactive hydrogen atoms, such as glycerol, that is polyether polyurethanes, have found limited application in such composite systems because at least one of (1) fused surface layers would not solidify into a bond, (2) expensive additives were required to obtain adhesion, (3) most of these additives would negatively impact either the polyurethane foam hydrolytic stability, (4) or would cause excessive volatile organic compounds, (5) or would generate fogging upon aging.
There have been attempts to solve these problems, for instance U.S. Pat. No. 3,205,120 (Flanders, Sep. 7, 1965) discloses heat sealed urethane foam laminates that are prepared from the less expensive polyether urethane foam. This patent discloses that in an otherwise conventional commercial polyether flexible urethane foam reaction mixture, a minor amount of a low molecular weight polyol selected from the group consisting of a polyoxyalkylene polyol, a hydroxyaliphatic ester of a phosphorus-containing acid and a hydroxyl-containing natural oil was included. The polyol additives and/or the hydroxyl-containing natural oils claimed in U.S. Pat. No. 3,205,120 have, however, low molecular weights, thus resulting in foam tightening and poor resiliency. Indeed such polyols are mainly used to produce rigid foams and have strong odor.
Another approach to solving the problems with flexible polyurethane foams in heat bonding has been to use polyester polyols, especially when they are made using suitable auxiliary agents and additives. The chemistry of these polyester polyols and production of foams from them are described in “Polyurethane Handbook” by G. Oertel et al. Hanser publishers. Often however, these foams can be produced only on special high pressure machines because polyester polyols have high viscosities, hence are difficult to mix with the isocyanates and other formulation components. In addition, foams made therefrom are inferior to polyether polyurethanes in some of their properties such as the openness of their cells, their elasticity, their resistance to moisture and heat or a combination thereof. Furthermore polyester polyurethanes are usually made from less readily available, thus more expensive, materials than conventional polyether polyols. Addition of polyester polyols, especially those containing aromatic rings, can also be added to conventional polyether polyols to get flame-laminable polyurethane foams as claimed in U.S. Pat. No. 6,638,990 for instance.
A third class of polyols to produce polyurethane products are the polyether-ester and/or polyester-ether polyol types. These polyols are either polyether polyol which are subsequently esterified with and acid or a lactone, or polyester polyols which are reacted with alkoxides. For instance, DE 2110276 describes polyols made by the second process which also contain nitrogen atoms to get foam adhesion by flame lamination or dielectric lamination. However all examples still contain a flame retardant in their formulations, hence will generate VOC's.
The disadvantages previously described of polyether polyurethane foams (which explain why polyester polyurethane foams are mostly used for flame bonding and ultrasound bonding), there is a great demand for a polyurethane foam which is reliable in production and capable of being flame bonded, ultrasound bonded or otherwise heat bonded.